Macro in blender? Quick way to set 1 hotkey for "Rotate 90 degrees" in UV editor?
I don't think there's a built in way to do this, so a while ago I created an add-on called UV Rotate Shortcuts.

This adds Shift+R and Ctrl+Shift+R to rotate the selected UVs by 90 degrees or -90 degrees respectively.

The rotation amount can be customised in the add-on's user preferences.